1. Field of the Invention
This invention pertains generally to lithography, microscopy and overlay methods. More particularly, it relates to application of the filtering and discriminating properties of biaxial crystals in microscopy, overlay, lithography of ring shapes and contact holes, lithographic illumination systems, and related fields.
2. Description of Related Art
Many scientific and industrial applications may be reduced to the fundamental optical problem of detecting a light point and measuring its lateral position. In many cases, a large background is superimposed on the feature to be detected.
Optical technologies are spread in numerous domains, including Lithography, Microscopy, Telecoms and Astronomy to name only a few. However, despite the broad array of applications, only a relatively small number of generic effects are available. An engineering tool using unique effects in light propagation may therefore open new avenues for devices in any of the derived applications and domains.
Conical refraction, light propagation in biaxial crystals along an optical axis, is a well known phenomenon that dates back a number of years. However, it does not have applications in modern technologies, as only recently suitable crystals with large conical effects have been developed (Berry, M. V., Conical diffraction asymptotics: fine structure of Poggendorff rings and axial spike. Journal Of Optics A-Pure And Applied Optics, 2004. 6(4): p. 289-300, incorporated herein by reference in its entirety).
Automated microscopy systems perform automatic identification, counting and position measurements of microscopic objects. The object may be a single point or a simple geometrical pattern. It may emit light or be a physical feature which affects the absorption, amplitude or phase of a transmitted, reflected or diffracted light. Some examples are fluorescent markers which are light self-emitting single points and fiducial cross marks on a semiconductor wafer which are phase lithographic features.
Several methods are known to retrieve the spatial position of one or several optical features. These methods translate the object's 2D light distribution into a two-dimensional pattern imaged on a CCD sensor. Adaptive algorithms turn the digitized image into a 2D pattern.
The simplest and most prevalent method is the direct imaging of the feature and the use of machine vision algorithms to retrieve the position. However, diffraction limits and the lack of field depth restrict the measurable volume.
Three general types of optical microscopes are used in science and industry.
Imaging microscopes—either brightfield or darkfield—image the object intensity with high magnification on a detector. In darkfield microscopy, the direct light is blocked and light from the specimen at oblique angles forms a bright image after reflection and diffraction.
Polarization and phase microscopes regroup a large family of microscopes which do not retrieve a direct amplitude image but rather retrieve an enhanced image. The most common types are: phase contrast microscopes and differential interference contrast (DIC) microscopes. Phase contrast microscopy, translates small variations in phase into changes in amplitude. It can be applied to living cells, thin tissue slices and lithographic patterns. Differential interference contrast (DIC) microscopy is a beam-shearing interference system in which the reference beam is sheared by a minuscule amount, generally somewhat less than the diameter of an Airy disk. In reflected light microscopy, optical path differences are created by discontinuities.
Confocal microscopes section the volume into thin slices and retrieve each slice separately. The key to the confocal approach is the use of spatial filtering to eliminate out-of-focus light or flare in specimens that are thicker than the plane of focus. This is done by imaging the object point by point through a pinhole.
Overlay registration refers to the relative alignment of two layers in a thin film photolithographic process. Overlay metrology is a major challenge in Lithography, for any lithographic technique. New solutions have to be developed to meet the stringent requirements of next-level lithography.
Previous methods had relied, up to 130 nm, on incoherent imaging with high magnification of fiducial marks positioned on each one of the layers. The relative position was retrieved using high accuracy machine vision algorithms. One widely used mark is the BiB, the “box in box” mark.
For the latest nodes, several new technologies have been developed. For example, grating marks and diffractive marks have been developed. These marks have been evaluated by manufacturers, selected users, NIST and SEMATECH, and the conclusion is that the main source of uncertainty in the newest technologies is the unmodeled residuals of the process.
Several RET—Resolution Enhanced Techniques—have been developed in order to improve the resolution of Lithographic systems. The major RET techniques are: OPC, optical proximity correction, OAI, (off-axis illumination) and PSM (phase shift masks). OAI, is a standard production tool for resolution enhancement. When the illumination falls on the mask at angles adapted to the pitch of periodic structures in the layout, the imaging characteristics of these periodic features are significantly enhanced. The on-axis components of the image, which do not add contrast, are reduced or eliminated. Another technique, IIL (Imaging Interferometric Lithography), adds a reference through an additional beam.
In general, a uniform wave illuminates a mask with an image of thin features, as lines in the vertical or horizontal direction. The structure, which for simplicity may be a linear grating, creates diffracted orders, at angles depending on the grating spacing. These orders are collected by the lens. At its aperture, or Fourier plane, the DC component and the diffracted orders will be represented by points, with positions proportional to the spatial frequency. We will refer to this plane as the frequency plane. At the frequency plane—or aperture—a physical or virtual stop represents the limit on numerical aperture of the specific lens.
If the diffracted orders are able to pass the stop, the image is reconstructed with adequate details at the image plane. If not, only the DC component is transmitted and a uniform light distribution is created, erasing all spatial information. At the frequency plane, the distance between the DC component and the diffracted orders is proportional to the spatial frequency. High spatial frequencies—very dense features—will extend in the Fourier plane above the stop of the lens and will be lost, or will necessitate a higher Numerical Aperture lens. In on-axis illumination, the DC component passes at the center of the stop and the diffracted orders are located symmetrically at ±(fx,fy), with fx and fy being the spatial frequencies of the pattern.
By tilting the illuminated wave, OAI, Off-Axis Illumination, translates the angle of the DC component, and its position in the frequency plane as well as the angle of the diffracted orders by the same amount. In the Fourier plane the points representing the diffracted orders are shifted by an amount equal to the off-axis angle multiplied by the lens focal length. Due to the inherent quadracity of the intensity, the image can be reconstructed with high fidelity using the DC component and a single diffracted order, even if the conjugate order had been lost.
OAI permits to cover a much larger range of spatial frequencies. Its theoretical limit will be at the point that the DC component is at one edge of the Fourier plane domain and the diffracted order is at the other one, permitting close to double the frequency to be imaged with a given lens.
OAI permits to cover a frequency range centered on an off-axis point (gx,gy) with gx and gy being the spatial frequencies representing the off axis angle. Several configurations are available for OAI: the main ones are the dipole, the quadrupole, the annular aperture-and the quasar. The OAI is limited to angles inside the Numerical Aperture due to the requirement that the DC component will be transmitted in order to provide an interfering term.
Accordingly, an object of the present invention is a new approach to lithographic techniques and overlay registration of semiconductor wafers.